A machine constructed to handle mixed mail generates a number of very severe problems. By mixed mail is meant envelopes of varying sizes, shapes, and thicknesses, and unflapped (sealed) and flapped (unsealed) envelopes.
For the postage printing operation, the envelopes must be registered, by which is meant that the edge of the envelope, typically at the flapped side, must occupy a predetermined position so that the postage printer prints the postage at the proper location at the corner of the envelope. This is typically accomplished by positively pushing the envelope up against a registration surface as it is conveyed through the machine. When the envelope is conveyed horizontally or flat, along a machine deck, the registration surface is typically a vertical wall. When the envelopes are all of the same width, and are conveyed downstream with a short side leading, registration is easily accomplished by providing a second wall spaced by the envelope width from the registration wall. This solution cannot be implemented with envelopes of different widths. Another possible solution is to devise some form of tamper for tamping the opposite long edge of the envelope as it is fed through successive modules of the machine and thus push the registration edge up against the registration wall. But this solution has been far too expensive to implement as a reliable means for maintaining registration of envelopes whose width can vary. Moreover, when the envelopes are being transported through the machine at up to four per second, devising a tamper that can instantaneously adjust to the differently sized envelopes flying through has proved essentially impossible.